Denitrification-nitrification process in permeable coastal sediments: An investigation on the effect of salinity and nitrate availability using flow-through reactors

Permeable coastal sediments act as a reactive node in the littoral zone, transforming nutrients via a wide range of biogeochemical reactions. Reaction rates are controlled by abiotic factors, e.g., salinity, temperature or solute concentration. Here, a series of incubation experiments, using flow-through reactors, were conducted to simulate the biogeochemical cycling of nitrate (NO₃⁻) and phosphorus (P) in permeable sediments under different NO₃⁻ availability conditions (factor I) along a salinity gradient (admixture of river and seawater, factor II). In an oligotrophic scenario, i.e., unamended NO₃⁻ concentrations in both river and seawater, sediments acted as a permanent net source of NO₃⁻ to the water column. The peak production rate occurred at an intermediate salinity (20). Increasing NO₃⁻ availability in river water significantly enhanced net NO₃⁻ removal rates within the salinity range of 0 to 30, likely via the denitrification pathway based on the sediment microbiota composition. In this scenario, the most active removal was obtained at salinity of 10. When both river and seawater were spiked with NO³, the highest removal rate switched to the highest salinity (36). It suggests the salinity preference of the NO₃⁻ removal pathway by local denitrifiers (e.g., Bacillus and Paracoccus) and that NO₃⁻ removal in coastal sediments can be significantly constrained by the dilution related NO₃⁻ availability. Compared with the obtained variation for NO₃⁻ reactions, permeable sediments acted as a sink of soluble reactive P in all treatments, regardless of salinity and NO₃⁻ input concentrations, indicating a possibility of P-deficiency for coastal water from the intensive cycling in permeable sediments. Furthermore, the net production of dissolved organic carbon (DOC) in all treatments was positively correlated with the measured NO₃⁻ reaction rates, indicating that the DOC supply may not be the key factor for NO₃⁻ removal rates due to the consumption by intensive aerobic respiration. Considering the intensive production of recalcitrant carbon solutes, the active denitrification was assumed to be supported by sedimentary organic matter.